The problem of determining the identities and exact spatial arrangement of the atoms surrounding any given atom in a molecule is extremely important, and is fundamental to understanding the properties of any type of liquid, gas, or solid. In the case of materials with long range order, such as perfect crystals, this information can often be obtained with X-ray or particle beam diffraction techniques. Such diffraction techniques rely on the fact that all of the atoms in a perfect lattice reside at fixed, periodic distances from any given atom, and that this periodicity is retained regardless of how far one moves within the lattice from the atom in question.
For materials without long range order, the diffraction techniques are far less useful; one can determine local configuration in this way only for relatively simple molecules composed of a single element. Considerable insight or more complicated molecules can often be gained from optical spectroscopy and magnetic resonance techniques. However, these techniques suffer from the drawback of providing only indirect evidence, from which the structural parameters of interest for any given molecule must be inferred.
Many of these limitations can be overcome with the recently developed technique of Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy (1,2). In EXAFS spectroscopy, the X-ray absorption coefficient of a material is measured as a function of energy from the K-edge or L-edge of a specific element in the material to as far as 1000 electron volts above the edge. The absorption of X-rays by the element is accompanied by the ejection of photoelectrons, which can be scattered from neighboring atoms. Back-scattering of these photoelectrons from atoms in the immediate vicinity of the absorbing atom gives rise to a periodic "wiggle" structure in the X-ray absorption spectrum (1,3,4). By analyzing this "wiggle" structure above the absorption edge of a particular element, information can be obtained about the spatial arrangement of atoms in the immediate vicinity of the absorbing species. Since only the nearby atoms are involved, long-range order is not required; therefore, the EXAFS technique can by applied to the study of a broad class of materials, including liquids, gases, and amorphous or crystalline solids.
In the past, chemical structure research with the EXAFS technique has been limited by the lack of suitably intense sources of X-rays. This deficiency is now being remedied to some extent by the increasing availability of synchrotron radiation, which is being harnessed in a number of X-ray test facilities throughout the world. There are certain types of EXAFS experiments, however, which cannot be performed easily with syncrotron X-ray sources. Most significant, perhaps, are those experiments which are designed to analyze highly transient structures such as chemically reactive intermediates or the excited electronic states of molecules. These experiments could be carried out if it were possible to obtain a complete EXAFS spectrum with a single, intense, short pulse of X-rays synchronized with the optical or electrical excitation of the sample. Previous work in our laboratories has indicated that laser-produced plasmas should be nearly ideal X-ray sources for experiments of this type.
Typical apparatus according to the present invention for obtaining EXAFS data of a material comprises means for directing radiant energy from a laser onto a target to produce X-rays of a selected spectrum and intensity at the target, means for directing X-rays from the target onto spectral dispersive means so located as to direct the spectrally resolved X-rays therefrom onto recording means, and means for positioning a sample of material in the optical path of the X-rays. Typically the radiant energy is directed to the target in a single pulse in such manner as to produce soft X-rays from the target suitable for obtaining the EXAFS spectrum of the material, which typically is an element having an atomic number of less than 40.
The apparatus may comprise also means for moving the surface of the target, in which case the radiant energy typically is directed to the moving target surface in a series of pulses in such manner as to produce soft X-rays from the target suitable for obtaining the EXAFS spectrum of the material.
The X-rays from the target preferably comprise continuum radiation in a selected EXAFS spectral regime of the sample. Typically the target comprises essentially an element having a continuum just above the L-lines that includes a selected EXAFS spectral regime of the sample. Or the target may comprise a plurality of elements whose lines are spaced closely enough to form virtually a continuum in a selected EXAFS spectral regime of the sample. Such a target typically comprises a mixture of elements of adjacent atomic numbers.
The radiant energy typically comprises a laser pulse with a power density of at least about 10.sup.13 watts per square centimeter, and the target tyically comprises a solid (typically metal) surface, whereby a surface plasma is formed and raised to the kilovolt temperature regime. The laser pulse preferably is focused to strike a focal spot on the target about 10 to 1000 micrometers in diameter.
The means directing the X-rays from the target typically comprises a baffle having an aperture through which the X-rays can proceed toward the spectral dispersive means, which typically comprises a crystal monochromator. Advantageously the means directing the X-rays from the target directs one portion of them onto the sample of material and an adjacent portion of them alongside the sample.
Typically the recording means comprises a photographic film capable of providing a visible representation of the EXAFS data, and the sample of material comprises a film located in the path of only a portion of the X-rays throughout a selected spectral band so that the X-rays directed onto the photographic film form two separate images thereon comprising a reference spectrum representative of a portion of the X-rays throughout the selected band that was not affected by the sample and an absorption spectrum representative of a portion of the X-rays throughout the selected band that was modified by transmission through the sample.
The spectral dispersive means typically comprises either a Bragg reflector or a diffraction grating, either flat or curved.
The laser pulse typically has a width of less than about 10 nanoseconds; in which case the sample of material may be in a highly transient state, typically comprising a chemically reactive intermediate, molecules with excited electronic states, or other highly transient spatial arrangement of atoms.